Title:Detection of an asymmetry in the envelope of the carbon Mira R Fornacis using VLTI/MIDI

Abstract: Aims. We present a study of the envelope morphology of the carbon Mira R For
with VLTI/MIDI. This object is one of the few asymptotic giant branch (AGB)
stars that underwent a dust-obscuration event. The cause of such events is
still a matter of discussion. Several symmetric and asymmetric scenarios have
been suggested in the literature. Methods. Mid-infrared interferometric
observations were obtained separated by two years. The observations probe
different depths of the atmosphere and cover different pulsation phases. The
visibilities and the differential phases were interpreted using GEM-FIND, a
tool for fitting spectrally dispersed interferometric observations with the
help of wavelength-dependent geometric models. Results. We report the detection
of an asymmetric structure revealed through the MIDI differential phase. This
asymmetry is observed at the same baseline and position angle two years later.
The observations are best simulated with a model that includes a uniform-disc
plus a Gaussian envelope plus a point-source. The geometric model can reproduce
both the visibilities and the differential phase signatures. Conclusions. Our
MIDI data favour explanations of the R For obscuration event that are based on
an asymmetric geometry. We clearly detect a photocentre shift between the star
and the strongly resolved dust component. This might be caused by a dust clump
or a substellar companion. However, the available observations do not allow us
to distinguish between the two options. The finding has strong implications for
future studies of the geometry of the envelope of AGB stars: if this is a
binary, are all AGB stars that show an obscuration event binaries as well? Or
are we looking at asymmetric mass-loss processes (i.e. dusty clumps) in the
inner part of a carbon-rich Mira?